Drilling and production operations for the exploration and production of offshore minerals require a floating platform that is as stable as possible against environmental forces. To avoid fatigue on the risers, some floating platforms (e.g., Spars) have drilling and production risers that are supported by buoyancy xe2x80x9ccansxe2x80x9d attached to each of the individual risers. As the water depth increases, larger buoyancy cans are used to support the in-water weight of the risers. Larger buoyancy cans require larger center-wells, which, in turn, increase the hull diameter. Increasing the hull diameter increases the hydrodynamic environmental loads acting on the platform. A larger mooring system is then required to withstand the increased environmental loads. These undesirable effects increase the fabrication and installation costs.
With present buoyancy can riser support systems, as the hull displaces laterally in response to environmental loads, the risers undergo a considerable amount of downward motion, or pull-down, with respect to the hull. Counterbalancing these environmental loads is crucial in order to avoid destruction of the risers or the platform. Counterbalancing environmental loads by tying the risers to a single table, gimballing the risers, or both, provide some additional support, but such systems still cannot support large tensile loads possible in offshore environments.
A need, therefore, exists for an improved apparatus, method and system that support drilling and production risers.
The described problems in supporting drilling and production risers are addressed by an apparatus, method and system having a dedicated riser tensioner for each riser, thereby allowing each riser to move vertically independently of the other risers without requiring gimballing. Further, the invention cross-couples the dedicated riser tensioners as a way to soften motions other than heave.
According to one aspect of the present invention, an apparatus is provided for supporting risers in a floating platform. The apparatus comprises a table disposed above a hull-top surface, a first riser opening in the table, and a first dedicated riser tensioner attached to the table and disposed about the first riser opening. The apparatus further comprises a second riser opening, and a second dedicated riser tensioner attached to the table and disposed about the second riser opening.
According to another aspect of the invention, a method is provided for supporting risers in a floating platform. The method comprises tensioning a first riser with a first dedicated riser tensioner. In addition, the method comprises tensioning a second riser with a second dedicated riser tensioner, wherein the first dedicated riser tensioner is responsive to the second dedicated riser tensioner.
According to another aspect of the invention, a system is provided for supporting risers in a floating platform. The system comprises a means for tensioning a first riser and a means for tensioning a second riser, wherein the means for tensioning a first riser is responsive to a means for tensioning a second riser.